nginx-1.13.4: use preprocessor define instead of const for comparison (#1483)
nginx-1.13.4: use preprocessor define instead of const for comparison (#1483)

Address a mistake I made in https://github.com/pagespeed/ngx_pagespeed/pull/1458

While testing the draft for 1.12.34.3 I noticed that older nginx
versions would fail to compile. Switch kNginx13_1_4 to a #define so we
can use it to properly compare.


pagespeed/ngx_pagespeed,pagespeed/ngx_pagespeed